Bump version to 6.15.3

• Diff: https://gitlab.com/gitlab-org/gitlab-qa/compare/v6.15.2...d06d74a82ba8de7fbbe63f89db62ac6fab74812d

• Release notes:

### Other changes (CI, backstage)

- !610 Relate QA issue and test session report
- !589 Remove high-capacity tag from CI Geo jobs

• Checklist before merging:

  • Diff link is up-to-date.
  • Based on the diff, lib/gitlab/qa/version.rb is updated, according to SemVer.
  • Release notes are accurate.

• Checklist after merging:

  • Update the release notes for the newly created tag.